Pad short string (Exercise)

using System;
using System.Text.RegularExpressions;

namespace ConsoleTests
{
    class ConsoleTests
    {
        static void Main()
        {
            string input;
            bool isValid;
            do
            {
                Console.Write("Enter string (20 characters max): ");
                input = Console.ReadLine();
                isValid = input.Length > 0 && input.Length <= 20;
                if (isValid == false)
                {
                    Console.WriteLine("Invalid string");
                }
            } while (isValid == false);
            Console.WriteLine("Result: {0}",input.PadRight(20,'*'));
        }
    }
}